1. Field of the Invention
The present invention generally relates to a transfer unit and an image forming apparatus, and more specifically, to a transfer unit and an image forming apparatus including a plurality of image carriers and an endless transfer belt.
2. Background Technologies
Recently, downsizing of an image forming apparatus has been emphasized. Technologies for the above purpose include the image forming apparatus that adopts a method by which a toner image for each color on a plurality of image carriers are sequentially transferred onto an intermediate transfer belt, and then are transferred to a transfer medium, in one process.
An example of the conventional image forming apparatus is explained in FIG. 5. The image forming apparatus in the example includes an image creating unit, including four photoreceptors or photoreceptors 11, 12, 13, and 14, and in which a tension of an intermediate transfer belt 10 is maintained by a driving roller 21, a tension roller 22, and an opposing secondary transfer roller 19. A full-color image is formed by sequentially transferring each color onto the intermediate transfer belt 10 by applying a predefined voltage from an upstream of a driven direction of the intermediate transfer belt 10 to primary transfer rollers 15, 16, 17, and 18. The formed full-color image is transferred to a transferred material or a sheet 24, by applying the predefined voltage to a secondary transfer roller 20, and then is fixed (not shown) and output. A residue of the toner that remains on the intermediate transfer belt 10 after the image transfer by the secondary transfer roller 20 is collected by a cleaning unit 23.
In the image forming apparatus above, an image reading sensor is disposed to improve an accuracy of a transfer position for each color transferred sequentially to the intermediate transfer unit by detecting gaps present between the positions of the transferred colors. Conventionally, the image reading sensor is disposed facing against a fixed roller such as a driving roller and a driven roller, for ensuring the accuracy in detection.
Japanese Patent Application Laid-Open No. 2004-326085 describes an image reading sensor disposed facing against an intermediate transfer belt, for constantly reading a toner image on the intermediate transfer belt with high accuracy, and easily positioning the image reading sensor.
Other technologies have been proposed for downsizing the device. Japanese Patent Application Laid-Open No. 2002-351228 describes an integration of a tension roller and a cleaning unit of an image forming apparatus including a plurality of image carriers, a belt member contacting the image carriers, a tension roller that maintains a tension of the belt member, a biasing unit that applies tension to the tension roller, a holding unit that supports the tension roller by an axis enabling the tension roller to freely rotate, and a cleaning unit for the belt member installed in the holding unit that contacts the tension roller via the belt member.
Japanese Patent Application Laid-Open No. 2005-010701 describes an image forming apparatus in which a tension roller and a detecting unit that detects a quantity of elongation of a transfer belt are integrated. Japanese Patent Application Laid-Open No. 2004-29057 describes an image forming apparatus in which a recording paper to which a toner image is transferred is fixed by applying heat and a pressure with a fixing device disposed at a relatively upper position to a secondary transfer roller in a transfer path of a vertical path configuration, while a lower plane of the intermediate transfer belt is formed facing against photoreceptor drums and disposed in an inclined manner with the secondary transfer roller side downward.
Japanese Patent Application Laid-Open No. 2004-326085 limits a positioning of an image reading sensor, requires a roller for the image reading sensor, and may require a unit that moves a secondary transfer roller to and away from the intermediate transfer belt, causing an issue of a size enlargement of the intermediate transfer unit. Japanese Patent Application Laid-Open Nos. 2002-351228, 2005-010701, and 2004-29057 do not consider the image reading sensor as an approach relating to downsizing of the image forming apparatus, causing an issue that the device including the image reading sensor is unable to achieve sufficient downsizing.